1. Field of the Invention
The present invention relates to a printing member for electrostatic photocopying, such as printing drum or plate.
2. Description of the Prior Art
Printing members for electrostatic photocopying are used to form on copying paper a visible image pattern corresponding to a photo or light image of the pattern to be copied in the manner described below.
The photocopying process starts with electrically charging the surface of the printing member uniformly all over it, onto which a photo or light image of the pattern to be copied is projected to form an electrostatic latent image. Then a toner powder is applied to the charged surface of the printing member for developing the latent image, and copying paper is pressed against the surface of the printing member to print a visible image pattern on the copying paper.
There has heretofore been proposed a printing member for electrostatic photocopying which comprises a substrate having a conductive surface and a photoelectric-sensitive, electrically chargeable layer formed on the conductive surface of the substrate. The photo-electrically-sensitive, electrically chargeable layer has a P type first non-single-crystal semiconductor layer, a first laminate member having an I type second non-single-crystal semiconductor layer and an I type third non-single-crystal semiconductor layer having a smaller energy band gap than that of the second non-single-crystal semiconductor layer, and an I type fourth non-single-crystal semiconductor layer having equal to or larger energy band gap than that of the second non-single-crystal semiconductor layer. In this instance, a P type first non-single-crystal semiconductor layer is formed of amorphous silicon (Si). The I-type second and third non-single-crystal semiconductor layers are formed of amorphous silicon and Si.sub.1-x Ge.sub.x (0&lt;x&lt;1), respectively,
With the conventional printing member of such a structure, the surface of the I-type non-single-crystal semiconductor layer of the chargeable layer forms the printing surface. When the chargeable layer, with its printing surface electrically charged positive, is exposed to a photo or light image of a pattern charged positive, is exposed to a photo or light image of a pattern to be copied, the light is absorbed mainly by a laminate member of the I-type second and third non-single-crystal semiconductor layers, with the result that photo carriers formed by electron-hole pairs are created chiefly in the laminate member. The electrons flow through the I-type fourth non-single-crystal semiconductor layer, neutralizing the positive charges on the printing surface. On the other hand, the holes flow through the P-type non-single-crystal semiconductor layer and reach the conductive surface of the substrate. In this way, the latent image pattern corresponding to the photo image pattern is formed in the conductive layer through such a mechanism as mentioned above. The two I type second and third non-single-crystal semiconductor layers of the laminate member by which the applied light is mainly absorbed have different energy band gaps of 1.7 to 1.9 and 1.2 to 1.5 eV, respectively. This permits the use of light over a wide range of wavelength from visible light to laser light for applying the photo or light image pattern to the chargeable layer.
In the case of the conventional printing member, however, the semiconductor layers making up the chargeable layer are all made of amorphous silicon and the I-type second and third non-single-crystal semiconductor layers are constituted of amorphous silicon and Si.sub.1-x Ge.sub.x (0&lt;x&lt;1), respectively. Since the Si.sub.1-x Ge.sub.x forming the I-type non-single-crystal semiconductor layer is poisonous, much care is necessary in handling the printing member. Moreover, the I-type third non-single-crystal layer can be produced by a CVD method through use of a material gas of GeH.sub.4, GeF.sub.4, or the like, but germanium for the material gas is not easily available and expensive; in addition, it is difficult to produce such a material gas. This means that the formation of the I-type third non-single-crystal semiconductor layer and accordingly the manufacture of the printing member is difficult, and the printing member becomes costly.